Re-healing cratonic mantle lithosphere after the world's largest igneous intrusion: Constraints from peridotites erupted by the Premier kimberlite, South Africa

Abstract

Constraining the processes that drive the disruption and re-healing of cratonic mantle roots is essential in understanding how cratons evolve. A striking example of this “cycle” of disruption and re-healing is provided by the deep mantle root currently observed beneath the ∼2.05 Ga Bushveld-Molopo Farms Complex (BMFC) of the central Kaapvaal Craton. Magmatism that produced the BMFC involved considerable disruption of the craton mantle root, yet the diamondiferous nature of the root, as sampled by Mesoproterozoic kimberlite eruptions, indicates that following Paleoproterozoic disruption, the lithosphere was re-healed and a cool geotherm re-established. A snap-shot of this mantle root provided by 48 peridotite xenoliths from the ∼1.2 Ga Premier (now Cullinan) kimberlite allows us to examine the evolution of the central Kaapvaal mantle lithosphere to constrain this re-healing process.Premier peridotites can be classified into 3 petrographic groups: an olivine-rich group (>80% olivine), with predominantly Archean Re-depletion (TRD) ages (Group I), a more orthopyroxene-rich group (Group II; <80% olivine) with dominantly Paleoproterozoic TRD ages, whose mode at ∼2.1 Ga matches the age of the BMFC, and a deformed, metasomatized group, with TRD ages mostly ∼2.1 Ga. Paleoproterozoic Group II and deformed peridotites represent 70% of the peridotite population and have systematically lower olivine Fo (molar 100×Mg/(Mg+Fe)) contents and higher Al2O3 compared to Archean Group I peridotites. Re-Os and other highly siderophile element (HSE) systematics indicate that the Paleoproterozoic TRD ages reflect the time of melt extraction. The dominant Group II peridotites have Al-Yb systematics indicative of a low-P melt-residue origin. The combined data rule out the suturing of the mantle root by accumulation of melting residues associated with the BMFC plume event. Instead, we invoke a model in which rapid decay of a plume-flux associated with BMFC generation leaves a lithospheric “gap” in the central craton that becomes filled with buoyant slab-derived oceanic lithospheric mantle associated with the subduction system that generated the ∼2.1 to ∼1.8 Ga western Kheis-Okwa-Magondi margin to the Kaapvaal Craton. An evolution to flat-slab subduction rapidly shuts down BMFC magmatism, emplaces Paleoproterozoic eclogites and diamonds in the root and heals the lithosphere disrupted by the plume. Such plume-subduction interaction, also observed in the modern Earth, might be a common process affecting the lithosphere of other cratons.